The present invention relates to a method for making a heat-resistant elastic article and a heat-resistant elastic article. The invention especially relates to a method of making elastic fibers and polymeric elastic fibers wherein the elastic fibers are capable of withstanding dyeing and heat-setting processes that typically are conducted at elevated temperatures (such as 110-230xc2x0 C. and especially at greater than or equal to 130xc2x0 C. for minutes). The inventive method comprises radiation crosslinking an article (or plurality of articles) under an inert or oxygen limited atmosphere (for example, in N2, argon, helium, carbon dioxide, xenon and/or a vacuum) wherein the article (or articles) comprises at least one amine stabilizer and preferably another optional stabilizer additive. More preferably, the radiation crosslinking is performed at a low temperature (xe2x88x9250 to 40xc2x0 C.). The elastic article (or articles) comprises a homogeneously branched ethylene interpolymer (preferably a substantially linear ethylene interpolymer), a substantially hydrogenated block polymer, or a combination of the two. The heat-resistant elastic articles (especially fibers) are useful in various durable or repeated-use fabric applications such as, but not limited to, clothing, under-garments, and sports apparel. The heat-resistant elastic fibers can be conveniently formed into fabrics using well-known techniques such as, for example, by using co-knitting techniques with cotton, nylon, and/or polyester fibers.
Disposable articles are typically elastic composite materials prepared from a combination of polymer film, fibers, sheets and absorbent materials as well as a combination of fabrication technologies. Whereas the fibers are prepared by well known processes such as spun bonding, melt blowing, melt spinning and continuous filament wounding techniques, the film and sheet forming processes typically involve known extrusion and coextrusion techniques, for example, blown film, cast film, profile extrusion, injection molding, extrusion coating, and extrusion sheeting.
A material is typically characterized as elastic where it has a high percent elastic recovery (that is, a low percent permanent set) after application of a biasing force. Ideally, elastic materials are characterized by a combination of three important properties, that is, a low percent permanent set, a low stress or load at strain, and a low percent stress or load relaxation. That is, there should be (1) a low stress or load requirement to stretch the material, (2) no or low relaxing of the stress or unloading once the material is stretched, and (3) complete or high recovery to original dimensions after the stretching, biasing or straining is discontinued.
To be used in the durable fabrics, the fibers making up the fabric have to be, inter alia, stable during dyeing and heat setting processes. We found that the polyolefinic fibers that were irradiated in air tended to fuse together when subjected to the high temperatures typical of dyeing processes (about 120xc2x0 C. for 30 min). Conversely, we surprisingly and unexpectedly found that when irradiated under an inert atmosphere, resultant crosslinked fibers were highly stable during the dyeing process (that is, the fibers did not melt or fuse together). The addition of a mixture of hindered phenol and hindered amine stabilizers further stabilized the fibers at heat setting condition (200-210xc2x0 C.).
Block polymers generally are elastomeric materials that exhibit excellent solid-state elastic performance attributes. But unsaturated block polymers such as, for example, styrene-butadiene-styrene triblock polymers, tend to exhibit mediocre thermal stability, especially in the molten state and poor UV stability.
Conversely, known partially hydrogenated (or partially saturated) styrene block copolymers (for example, KRATON G block copolymers supplied by Shell Chemical Company) are difficult to melt process and draw into fibers or films. In fact, preparation of fine denier fiber (that is, less than or equal to 40 denier) or thin film (that is, less than or equal to 2 mils) from partially hydrogenated or partially saturated block polymers is generally not possible at commercial fabrication rates. To overcome characteristic melt processing and drawing difficulties, partially hydrogenated block copolymers are commonly formulated with various additives such as oils, waxes and tackifiers. But in order to achieve good melt processability and drawability, very high levels of low molecular weight additives are typically required which tend to compromise strength and elastic properties.
Lycra(trademark) (trademark of Dupont Chemical Company), a segmented polyurethane elastic material, is currently used in various durable fabrics. But a shortcoming of Lycra is that it is not stable at typical high heat setting temperatures for PET fiber (200-210xc2x0 C.). Similar to ordinary uncrosslinked polyolefin-based elastic materials, Lycra articles tend to lose their integrity and shape and elastic properties When subjected to elevated service temperatures. As such, Lycra can not be successful used in co-knitting applications with high temperature fibers such as polyester fibers. Another major shortcoming of Lycra is its cost. That is, Lycra tends to be extremely cost prohibitive for many of applications.
WO 99/63021, the disclosure of which is incorporated herein by reference, describes elastic articles comprised of a substantially cured, irradiated, or crosslinked (or curable, irradiated or crosslinkable) homogeneously branched ethylene interpolymer characterized as having a density less than 0.90 g/cm3 and containing at least one nitrogen-containing stabilizer. The described elastic articles are disclosed as suitable for use in applications where good elasticity must be maintained at elevated temperatures and after laundering such as, for example, elastic waist bands of undergarments and other clothing. WO 99/63021 also generally teaches that the nitrogen-containing stabilizer can be used in combination with phenolic and phosphite stabilizers and reported examples therein are known to include a combination of amine, phenol and phosphorus-containing stabilizers. But there is no description of crosslinking or irradiation under an inert or reduced oxygen atmosphere and there is no specific teaching of improved heat-setting and high temperature dyeing performance.
U.S. Pat. No. 5,324,576, the disclosure of which is incorporated herein by reference, discloses an elastic nonwoven web of microfibers of radiation crosslinked ethylene/alpha olefin copolymers, wherein a substantially linear ethylene polymer (that is, INSITE technology polymer XUR-1567-48562-9D from The Dow Chemical Company) is set forth in the reported inventive example. The substantially linear ethylene polymer is subjected to electron beam radiation in a nitrogen inerted chamber at an oxygen level of approximately 5 ppm. While the substantially linear ethylene polymer is known to contain 500 ppm of a phenolic antioxidant, there is no teaching to add a nitrogen-containing stabilizer to the polymer. Moreover, there is no disclosure regarding the elastic performance of the radiated substantially linear ethylene polymer at elevated temperatures.
Chemical abstract N1993:235832 (D. W. Woods and I. M. Ward, Plast., Rubber Comps. Process. Appl. (1992), 18(4), 255-61), the disclosure of which is incorporated herein by reference, describes the use of radiation under nitrogen to crosslink HDPE fiber to improve creep resistance.
WO 99/60060, the disclosure of which is incorporated herein by reference, discloses heat resistant elastic fiber comprised of polyolefinic elastomers made using single site catalyst.
Elastic materials such as films, strips, coating, ribbons and sheet comprising at least one substantially linear ethylene polymer are disclosed in U.S. Pat. No. 5,472,775 to Obijeski et al., the disclosure of which is incorporated herein by reference. But Obijeski et al. do not disclose the performance of their elastic materials at elevated temperatures (that is, at temperatures above room temperature).
WO 94/25647, the disclosure of which is incorporated herein by reference, discloses elastic fibers and fabrics made from homogeneously branched substantially linear ethylene polymers The fibers are said to posses at least 50 percent recovery (that is, less than or equal 50 percent permanent set) at 100 percent strain. However, there is no disclosure in WO 94/25647 regarding the elasticity of these fibers at elevated temperatures or the effects of laundering on these fibers.
WO 95/29197, the disclosure of which is incorporated herein by reference, discloses curable, silane-grafted substantially ethylene polymers which are useful in wire and cable coatings, weather-stripping, and fibers. In the Examples, inventive samples include fibers comprising silane-grafted substantially ethylene polymers having densities of 0.868 g/cm3 and 0.870 g/cm3. The inventive examples are shown to exhibit improved elastic recovery at elevated temperatures.
U.S. Pat. No. 5,525,257 to Kurtz et al., the disclosure of which is incorporated herein by reference, discloses that low levels of irradiation of less than 2 megarads of Ziegler catalyzed linear low density ethylene polymer results in improved stretchability and bubble stability without measurable gelation. Kurtz et al. do not provide any disclosure regarding elasticity at elevated temperatures.
U.S. Pat. No. 4,957,790 to Warren, the disclosure of which is incorporated herein by reference, discloses the use of pro-rad compounds and irradiation to prepare heat-shrinkable linear low density polyethylene films having an increased orientation rate during fabrication. In the examples provided therein, Warren employs Ziegler catalyzed ethylene polymers having densities greater than or equal to 0.905 g/cm3.
Various compounds are disclosed in the art and/or sold commercially as high temperature stabilizers and antioxidants. However, the criteria employed to distinguish these compounds as stabilizers and antioxidants typically relates to their ability to resistance yellowing, crosslinking and/or the ill-effects of irradiation (for example, gamma irradiation for purposes of sterilization).
In other instances, different types of stabilizers are equated to one another or are said to perform comparably. For example, it is known that hindered phenolic stabilizers (for example, Irganox(copyright) 1010 supplied by Ciba-Geigy) can be as effective as hindered amine stabilizers (for example, Chimassorb(copyright) 944 supplied by Ciba-Geigy), and vice versa. In a product brochure entitled, xe2x80x9cChimassorb 944FL: Hindered Amine Light Stabilizer Use and Handlingxe2x80x9d, printed 1996, Ciba-Geigy states Chimassorb 944 xe2x80x9cgives long-term heat stability to polyolefins by a radical trapping mechanism similar to that of hindered phenols.xe2x80x9d
Further, there is some belief that there is no universally effective stabilizer for polymers as the definition for stability inevitably varies with each application. In particular, there is no effective stabilizer for washable, high temperature serviceable polyolefinic elastic materials.
In general, stabilizers are known to inhibit crosslinking. In regard to crosslinking generally, there are several disclosures relating to radiation resistant (for example, gamma and electron beam) polymer compositions comprising amine stabilizers. Such disclosures typically teach relatively high levels of amine stabilizer (for example, greater than or equal to 0.34 weight percent) are required where inhibition of crosslinking, discoloration and other undesirable irradiation effects are desired. Another examples include stabilized disposal nonwoven fabrics (see, for example, U.S. Pat. No. 5,200,443, the disclosure of which is incorporated herein by reference) and stabilized molding materials (for example, syringes). Gamma sterilization resistant fibers, including amine coatings and the use of hybrid phenolic/amine stabilizers are also known. See, for example, U.S. Pat. No. 5,122,593 to Jennings et al., the disclosure of which is incorporated herein by reference.
Stabilized polyethylene compositions with improved resistance to oxidation and improved radiation efficiency are also known. M. Iring et al. in xe2x80x9cThe Effect of the Processing Steps on the Oxidative Stability of Polyethylene Tubing Crosslinked by Irradiationxe2x80x9d, Die Angew Makromol. Chemie, Vol. 247, pp. 225-238 (1997), the disclosure of which is incorporated herein by reference, teach that amine stabilizers are more effective towards inhibiting electron-beam irradiation effects (that is, provide better resistance against oxidation) than hindered phenols.
WO 92/19993 and U.S. Pat. No. 5,283,101, the disclosures of which are incorporated herein by reference, discloses launderable retroreflective appliques comprised of a multicomponent binder composition consisting of an electron-beam curable elastomer, crosslinker(s), and coupling agent(s) and optional colorants, stabilizers, flame retardants and flow modifiers. The allegedly inventive appliques are said to be capable of withstanding ordinary household washing conditions as well as more stringent industrial washings without loss of retroreflectiveness. Illustrative examples of electron-beam curable elastomers of the binder are said to be xe2x80x9cchlorosulfonated polyethylenes, ethylene copolymers comprising at least about 70 weight percent of polyethylene such as ethylene/vinyl acetate, ethylene/acrylate, and ethylene/acrylic acid, and poly(ethylene-co-propylene-co-diene) (xe2x80x9cEPDMxe2x80x9d) polymers.xe2x80x9d Optional stabilizers are described to be xe2x80x9cthermal stabilizers and antioxidants such as hindered phenols and light stabilizers such as hindered amines or ultraviolet stabilizersxe2x80x9d. Although there is an equating of the suitability or effectiveness of hindered phenols to hindered amines in the descriptions of WO 92/19993 and U.S. Pat. No. 5,283,101, no stabilizer of any kind is exemplified in the provided examples. Further, although the applique can employ polymers that are described as xe2x80x9chighly flexiblexe2x80x9d before and after electron-beam curing, neither the selected polymers nor the applique itself are described as xe2x80x9celasticxe2x80x9d. While elastic materials typically have a high degree of flexibility (that is, Young""s Modulus of less than 10,000 psi (68.9 MPa) where lower modulus means more flexibility), highly flexible materials can be nonelastic as the terms xe2x80x9cnonelasticxe2x80x9d and xe2x80x9celasticxe2x80x9d are defined herein below. That is, not all xe2x80x9chighly flexiblexe2x80x9d materials are elastic.
Although there is an abundance of art related to elastic materials comprising curable, radiated and/or crosslinked ethylene polymers, and there is also an abundance of art related to stabilized ethylene polymer compositions and articles, there is no known disclosure of a polyolefinic elastic material with effective additive stabilization wherein the stabilization does not inhibit the desirable effects of irradiation and/or crosslinking (that is, impart elevated temperature elasticity) and yet does inhibit the loss of elastic integrity (that is, scission) when the material is subjected to processing or finishing steps at elevated temperatures.
Further, in a product brochure entitled, xe2x80x9cStabilization of Adhesives and Their Componentsxe2x80x9d, pp. 8-9 (1994), Ciba-Geigy, a premier stabilizer supplier, states that scission occurring in elastomeric materials(for example, styrene-isoprene-styrene block copolymers) at elevated temperatures above 70xc2x0 C. is not readily controlled by the use of antioxidants.
As such, there is a present need for cost-effective, stable elastic articles having good elasticity at elevated temperatures as well as good heat setting characteristics. That is, there is a need for elastic articles which in-service retain their shapes under strain at elevated temperatures (for example, greater than or equal to 125xc2x0 C.) and can be processed, finished and/or laundered at even higher temperatures and still retain their in-service elastic characteristics. There is also a need for a method of making elastic articles having good elasticity at elevated temperatures as well as good dyeing and heat setting characteristics. We have discovered that these and other objects can be completely met by the invention herein described.
We surprisingly discovered that the combination of radiation under an inert atmosphere or oxygen-reduced atmosphere (that is, less than 20 ppm, preferably less than 10 ppm, more preferably less than 5 ppm oxygen) and the use of an amine stabilizer such as a hindered amine or aromatic amine (and optionally a hindered phenol and/or a phosphorus-containing stabilizer) can provide elastic materials (especially fibers) that maintain their elasticity, yet are sufficiently crosslinked to confer sufficient heat resistance to permit high temperature dyeing and heat setting. The broad aspect of the invention is a method of making an elastic article having improved heat resistance (that is, a heat-resistant elastic article) comprising the steps of:
(a) providing at least one elastic polymer or elastic polymer composition (for example, a homogeneously branched ethylene interpolymer having a density of less than or equal to 0.90 g/cm3 at 23xc2x0 C. or a substantially hydrogenated block copolymer) which contains at least one amine or nitrogen-containing stabilizer therein,
(b) fabricating, forming or shaping the polymer or polymer composition into an article, and
(c) during or after the fabrication, forming or shaping, subjecting the article to ionizing radiation while the article is in or under an inert or oxygen-reduced atmosphere.
Preferably, the irradiation or crosslinking is effectuated using ionizing radiation, most preferably by using electron beam irradiation. Also, preferably, the article (for example, but not limited to, the extrudate, filament, web, film or part) is permitted to cool or is quenched to ambient temperature (that is, permitted to substantially solidify) after fabrication or formation before the application of ionizing radiation to effectuate irradiation or crosslinking. Most preferably, the irradiation is conducted at a low temperature.
An important benefit of the inventive fibers is now elastic fibers can be used in combination with fibers which require heat setting at elevated temperatures such as, for example, that PET fibers.